US3128325A - High temperature furnace - Google Patents

Description

A ril 7, 1964 J- C. ANDERSEN ETAL HIGH TEMPERATURE FURNACE Filed June 27. 1960 29 53 4s 47 IO 3 Sheets-Sheet 2 IN VENTORS JAMES C. ANDERSE-N Tl MOTHY J. KEATY ATTORNE Y J c. ANDERSEN ETAL 3,128,325

HIGH TEMPERATURE FURNACE April 7, 1 964 5 Sheets-Sheet 3 Filed June 27, 1960 INVENTORS JAMES C. AN DERSE N BYT'MOTHY J. KEATY ATTORNEY United States Patent 3,128,325 HIGH TEMPERATURE FURNACE James C. Andersen, Niagara Fails, and Timothy J. Keaty, Ransomville, N.Y., assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Filed June 27, 1960, Ser. No. 38,797 Claims. (CI. 13-20) heating elements have had an upper temperature limit of about 1550" Cl Service above this temperature, even for short periods of time, has involved an unacceptable sacrilice in heating element life.

The increasing importance of high temperature refractory materials has resulted in a growing demand for furnacescapable of operating substantially above 1650 C.

Accordingly it is an important object of the present invention to provide an electrically heated furnace capable of operating at temperatures up to 1850 C.

Another object is to provide a furnace utilizing silicon carbide electrical resistance heating elements capable of operation at temperatures up to 1850" C.

Still another object is to provide an electrical resistance heated furnace utilizing siliconcarbide heating elements which is capable of operation at temperatures up to 1850 C. and having a furnace chamber wherein a variety of inert or oxidizing atmospheres can be provided, as desired.

Other objects will become apparent to those skilled in the art by reference to the accompanying drawings and specification.

FIGURE 1 is an isometric view of a preferred embodiment of the furnace of the present invention, with portions being broken away in section;

FIGURE 2 is an enlarged longitudinal sectional view taken along the line 22 of FIG. 1;

FIGURE 3 is a reduced cross sectional view taken along the line 33 of FIG. 2;

FIGURE 4 is an enlarged detail sectional view showing the manner in which the furnace chamber is connected to the end wall of the casing of FIGS, 1, 2, and 3; and

FIGURE 5 is an isometric view of a second embodiment of the furnace of the present invention, with portions being broken away in section.

Briefly the furnace of the present invention comprises a hollow graphite mufiler of cylindrical configuration with closed ends, coaxially of which and extending therethrough is positioned ahigh temperature-resistant (i.e. refractory) furnace chamber forming tube. Radially spaced from the furnace chamber and parallel thereto are electrical and thermal conducting silicon carbide heating elements. Cold ends of the heating elements extend from each end of the muffle, being shielded in high temperature-resistant (i.e. refractory) tubes, which provide electrical insulation and an enclosure for a protective gas atmosphere.

A gas inlet tube is directly connected to a wall of the muffle for admission of an inert gas to protect thesilicon carbide heating elements against oxidation and consequent deterioration. The gas bleeds off around the shrouded cold ends, providingprotection for them also.

Preferably a gas-tight casing surrounds the furnace as sembly and is filled with an appropriate insulating material.

By reference to the drawings, it will be seen specifically that the preferred form of the furnace comprises a hollow cylindrical muffle 10, made of graphite, which is an eleczontal angles 14. The vertical angles extend downwardly to form support feet 15. The frame elements 12 are positioned inwardly of the ends of the casing 11. The casing frame also includes horizontally disposed angles 16 which extend beyond the frame elements 12 and are joined with end vertical angles 17. Horizontal angles 18 connect the ends of the upper and lower horizontal angles 16 to complete the framework. 2

The center portion of the top of the casing consists of a sheet metal plate 19 secured in position to the frame by bolts 19 and a gasket 19". This provides access to the interior for assembly of the furnace. Top end plates 19" are fastened by gas-tight welds. Bottom plates 20 are also welded to the frame, as are side plates 21. Partition walls 22 are fastened to the frame elements 12 in gas-tight relationship, as by welding. These are positioned inwardly of the ends of the casing to provide access chambers 23. End plates 24 are'attached to the end verticle angles 17 and horizontal angles 18. These are removably secured as by bolts 25 so that they can be removed to open the access chambers 23. The end plates 24-are fitted in gastight relationship by a gasket 26.

The muffie 10 is placed within the casing 11 and is supported centrally thereof by a U-shaped support comprising a base block 27, resting on the bottom of the casing, andtwo upright blocks 28. The hollow mufiie is provided with apertured ends 29. As best shown in FIG. 3 the ends are each'provided with a central aperture 30 posi tioned coaxially of the mufile. Also the ends are provided with a plurality of apertures 31 spaced equally circumferentially and radially from the axis of the muffle and concentric to the central aperture 30. It is to be noted in FIG. 2 that the aperture patterns at each end of the mufiie are identical and the apertures in each end are in aligned relationship.

As best shown in FIG. 2, an isolated furnace chamber forming tube 32 in the preferred form of a high density refractory tube of alumina is fitted into the central apertures 30 and extends through the mufile, with the ends thereof extending through aligned holes 33 provided in the partition walls 22 and also aligned holes 34 provided in the end plates 24. The clearance between the holes in the muffle and the partition walls and the alumina tube is made as a reasonably tight mechanical fit. However, the alumina tube is sealed in the holes 34 in the end plates, as shown in the enlarged sectional view of FIG. 4 to provide a gas-tight seal.

The combined cooling and sealing means comprises an annular washer 35 which is welded gas tightly as at 36 to the end plates 24 in aligned relationship to the aperture 34. A double-walled tubular sleeve 37 is fitted into the aperture of the washer and is gas tightly secured by welding 38. The tubular sleeve includes a cavity 39 through which a coolant such as water is circulated. Inlet and outlet tubes 40 and 41 are connected to the cavity 39 in coolant sealing relationship and are connected to a suitable source of coolant (not shown) to provide the necessary cooling. The tubular sleeve 37 is provided with an inwardly tapered mouth 42. The alumina tube 32 is fitted through the interior of the sleeve 37 and extends a short distance beyond the end thereof. A heat-resistant annular gasket 43 of pliable material is fitted between the alumina tube and the mouth 42 of the sleeve. A cap 44 is fitted over the end of the sleeve 37 as by threading and compresses the gasket 43 in gas-tight relationship between the alumina tube and the tapered mouth of the sleeve. High temperature-resistant (i.e. refractory) sbieldmg tubes, such as alumina tubes 45 are also fitted tightly in the remaining apertures 31 in the end walls 29 of the muflle and extend through and just beyond the partition walls 22 in appropriate apertures 46 provided therein. These tubes do not extend through the muffie as does the furnace chamber; instead, they are fitted into the ends of the muffle with one end approximately flush with the inside of the end wall of the mufiie. The apertures in the partition walls are formed in the same pattern as that of the end walls of the mufile and all aperture patterns are aligned.

Silicon carbide heating elements 47 are removably positioned in the alumina tubes 45 and slidably extend through these tubes and the muffle It These heating elements include a central heating section 48, with socalled cold ends 49 joined thereto. The heating sections are exposed within the interior of the muifie and are in surrounding, radiant-heat transfer relationship to the furnace chamber formed by tube 32. The cold ends lie outside of the mufiie but are partially encased within the alumina tubes :5. Electrical connections are made to exposed portions of the cold ends of the heating ele ments, which protrude into the access chambers for such purpose. Power is brought to terminals 50 from a suitable source thence to straps 5i? to heat the resistance elements 47.

As shown in FIGURES 1 and 3, a sight tube 51 extends through the casing 11 and is directly connected at its inner end to the mufiie Iii. The sight tube is shown as being provided with a sight glass 52 and a cap 53 threadably retaining the glass in position. A gas inlet tube 54- is joined to the sight tube and a selected inert gas is ad mitted therethrough to fill the interior of the inuflie. This gas surrounds the heating elements, protecting them from oxidation, and bleeds oif through the alumina tubes 45 surrounding the cold ends of the elements and also thereby protecting them from deterioration by oxidation.

The interior space 55 of the casing 11, surrounding the mufile it? and associated parts, between the partition walls 22, is filled with a particulate mass of suitable thermal insulating material such as carbon black. An outlet tube 56 permits the inert gas to exhaust from the casing.

A second embodiment of the furnace of the present invention is illustrated in FIGURE 5. This embodiment is essentially the same as the preferred embodiment of FIGURES 14, except that the casing 57 is fabricated of firebrick 58. In this embodiment the interior space of the casing surrounding the mufiie and associated parts is filled with a particulate mass of thermal insulating material comprising fine silicon carbide and carbon. The carbon addition retards oxidation of the silicon carbide and inhibits formation of low melting silica glass.

In this embodiment the alumina tubes 45 surrounding the heating elements 4'7 are notched into the bricks 58 of the end walls of the casing 57 to provide support and the cold ends .9 of the silicon carbide resistance heating elements extend through suitable holes provided in the bricks, which provide electrical insulation. Appropriate electrical connections are made to the exposed ends of the heating elements in the manner of the preferred embodiments described above. A blanket 60 of high temperature-resistant fibrous material, such as aluminum silicate fibers is placed over the insulation material when the casing is open to provide access to the interior for assembling the furnace therein.

The invention is highlighted by the following examples:

EXAMPLE I A furnace was constructed in accordance with the present invention having a dense alumina furnace tube 1 /8 inside diameter. The heating elements were 28" x 8" x The heating elements were regularly spaced around the exterior of ther" urnace tube about 3% inches on center therefrom. Heat transfer of the silicon carbide heating elements resulted in only a 55 C. differential between the surface temperature of the element and the interior of the furnace tube at 1850 C.

EXAMPLE II A furnace was constructed in accordance with the concept illustrated in FIGURES 1, 2, 3 and 4 of the drawings using carbon as an insulation around the muffie. A run employing low watt-loading and high nitrogen flow, resulted in 518 hours of continuous service at 1800 C. The furnace was returned to service merely by replacing the necessary heating elements.

The high thermal eificiency of the carbon insulation minimizes electrical power requirements. For example, an ambient temperature of 1750 C. can be maintained at a power input of 2.2 kw. This means that expensive high current transformers are not required. Power requirements for various operating temperatures are shown in Table I below.

1 1:Qurface loading in watts per square inch of nominal element radiating sur ace.

EXAMPLE III.-FURNACE PERFORMANCE Two furnaces of the type shown in FIGURE 5 of the drawings have been extensively used in laboratory work. One furnace was in continuous service for 150 hours at 1750 C. before heating element failure. The other furnace was operated intermittently for a total of hours between 1600 and 1850 C. before one of the elements failed. The intermittent service constituted 40 cycles from room temperature to operating temperature with heat-up times of two to four hours. Both furnaces were restored to service by simply replacing the necessary heating elements.

Gases which can be used to form the protective atmosphere in furnaces of the present invention include nitrogen, argon and helium. Nitrogen is preferred from an economic standpoint; however, in the temperature range 1350-1650 C., argon or helium is preferred, because nitrogen reacts with silicon carbide to form silicon nitride in this temperature range.

In operating furnaces of the present invention it is preferred to keep the watt-loading per unit area of nominal element radiating surface at a minimum, preferably less than 20-25 watts per square inch. This is to prevent overheating of the current carrying silicon carbide bridges which leads to premature element failure. This low wattloading is easily accomplished in accordance with the present invention because of the high thermal efiiciency of the carbon insulation, which was referred to above as surrounding the muffie within the embodiment of FIGURES l-4. Also the silicon carbide-carbon mixture utilized in FIGURE 5 is also highly efficient and provides for the desired low watt-loading.

Another insulating material, in addition to carbon and carbon-silicon carbide mixtures already referred to, is alumina. Alumina in the form of small, hollow spheres is particularly desirable.

While the preferred material from which the furnace chamber is fabricated is high density, non-porous alumina, other materials such as zirconia can be employed.

In addition to the alumina tubes which are utilized to encase the cold ends of the heating elements, silicon carbide tubes are contemplated for this purpose for reasons of economy. However, since silicon carbide is electrically conductive, short alumina sleeve or washer-like electrical insulators will be used at each end of the tube to provide the desired electrical insulating interior surfaces and thereby keep the heating element out of contact with the tube. 7

Although it has been mentioned above, a particular advantage of the furnace of the present invention is that when a heating element fails, the furnace does not have to be rebuilt to return it to operating condition. By virme of the tubes in which the fcold ends of the heating elements are supported, heating elements can be replaced merely by sliding them out of their position and placing a new element therein. Thus a furnace is provided which i is characterized by long and eflicient operation at severe temperatures greatly exceeding those heretofore known in the art.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention, following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention or the limits of the appended claims.

We claim:

1. An electrical resistance furnace comprising a hollow casing formed of structural material such as metal and having gas outlet means and an interior closed by end walls; a hollow muflie formed of thermal insulating refractory material such as graphite and arranged within said interior and having an interior closed by end walls, said casing and mufiie end walls being provided with alined central apertures and alined outer apertures surrounding said central apertures, an elongated tube formed of dense refractory material such as alumina and extending through said mufile and easing, said tube fitting tightly in said central apertures in said casing and muffle end walls and forming an isolated furnace chamber, elongated shielding tubes formed of refractory material such as alumina and extending between said casing and mufile end walls, said shielding tubes fitting tightly in said outer apertures in said casing and muflle end walls and provided with interior surfaces of electrical insulating material such as alumina, a particular mass formed of thermal insulating refractory material such as carbon and filling said casing interior around said muffle, furnace tube and shielding tubes, removable elongated heating elements formed of electrical conducting refractory material such as silicon carbide and slidably extending through said mufile and shielding tubes, said heating elements including central heating sections exposed within said rnufile interior in radiant heat transfer relationship with said furnace tube and cold end portions arranged within said shielding tubes and thereby electrically insulated from said mufile and mass, and a gas inlet tube directly connected to said muflle'interior for providing an inert atmosphere surrounding said central heating sections and for permitting said atmosphere to bleed off around said cold end portions through said shielding tubes in order to effectively protect said heating elements against oxidation, whereby said furnace is capable of operating at temperatures up to about 1850" C., with only about a 55 C. differential between the surface temperature of said heating elements and the interior of said furnace tube, with a surface loading of less than about 25 watts per square inch of nominal heating element radiating surface, and with a power input requirement of less than about 3 kilowatts.

2. An electrical resistance furnace as in claim 1 wherein said casing material is selected from the group consisting of metal and firebrick, said muflle material is graphite, said furnacetube material is alumina, said shielding tube material is selected from the group consisting of alumina and silicon carbide, said shielding tubes of silicon carbide incorporating internal insulators of alumina to form said internal surfaces and thereby electrically insulate said heating elements from said silicon carbide shielding tubes as well, said mass material is selected from the group consisting of carbon, a mixture of carbon and silicon carbide, and alumina, and said heating element material is silicon carbide. I g i 3. An electrical resistance furnace comprising a hollow substantially gas-tight casing formed of metal and having gas outlet means, a central interior closed by inner end walls and outer access chambers closed by outer end walls, a hollow muflie formed of graphite arranged within said central interior and having an interior closed by end walls, said inner and outer'end walls and said muffle end walls being provided with alined central apertures and said inner end walls and muffle end walls being provided with alined outer apertures surrounding said central apertures therein, combined sealing and cooling means fitting substantially gas tightly in said central apertures in said outer end walls, an elongated tube formed of dense alumina and extending through said mufiles and casing, saidtube fitting tightly in said central apertures in said inner end walls and muffle end walls and substantially gas tightly in said combined sealing and cooling means and forming an isolated furnace chamber, shielding tubes formed of alumina and extending between said inner end walls and muffle end walls and fitting tightly in said outer apertures therein, a particulate mass formed of carbon black and filling said central interior around said muffle, furnace tube and shielding tubes, removable elongated heating elements formed of silicon carbide and slidably extending through said muffle and shielding tubes into said access chambers, said elements including central heating sections exposed within said muffle interior in radiant heat transfer relation to said furnace tube and cold end portions arranged within said shielding tubes and thereby electrically insulated from said mufile, mass and inner end walls, and a gas inlet tube directly connected to said muflie interior for providing an inert atmosphere surrounding said central heating sections and for permitting said atmosphere to bleed off around said cold end portions through said shielding tubes in order to effectively protect said heating elements against oxidation, whereby said furnace is capable of operation at temperatures up to about 1850 C., with only about a 55 C. differential between the surface temperature of said heating elements and the interior of said furnace tube, with a surface loading of less than about 25 watts per square inch of nominal heating element radiating surface, and with a power input requirement of less than about 3 kilowatts.

4. An electrical resistance furnace as in claim 3 wherein each of said combined cooling and sealing means; includes a double walled tubular sleeve fitting substantially gas tightly in said central aperture in the corresponding one of said outer end walls, said sleeve having a cavity through which a cooling medium is circulated, a threaded outer surface at one end and an inwardly tapered mouth at said one end, a heat resistant gasket arranged between said mouth and said furnace tube, and an internally threaded cap compressing said gasket into substantially gas tight relationship between said mouth and furnace tube.

5. An electrical resistance furnace comprising a hollow casing formed of firebrick and having an interior closed by end walls, a hollow muffle formed of graphite and arranged within said interior and having an interior closed by end walls, said casing and muflle end walls being provided with alined central apertures and alined outer apertures surrounding said central apertures, an elongated tube formed of dense alumina and extending through said muffle and easing, said tube fitting tightly through said central apertures in said casing and mufile end walls and forming an isolated furnace chamber, elongated shielding tubes formed of alumina and extending between said casing and muflie end Walls and fitting tightly in said outer apertures therein, a particulate mass formed of a mixture of carbon and silicon carbide and filling said casing interior around said muffle, furnace tube and shielding tubes, removable elongated heating elements formed of silicon carbide and slidably extending through said mufiie and shielding tubes, said heating elements including central heating sections exposed Within said mufile interior in nadiant heat transfer relation to said furnace tube and cold end portions arranged in said shielding tubes and thereby electrically insulated from said muffle and mass, and a gas inlet tube directly conneeted to said mufiie interior for providing an inert atmosphere surrounding said central heating sections and for permitting said atmosphere to bleed ofif around said cold end portions through said shielding tubes in order to efiectively protect said heating elements against oxidation, whereby said furnace is capable of operation at temperatures up to about 1850 C. with only about a 55 C. differential between the surface temperature of said heating elements and the interior of said furnace tube, with a surface loading Olf less than about 25 watts per square inch of nominal heating element radiating surface, and with a power input requirement of less than about 3 kilowatts.

References Cited in the file of this patent UNITED STATES PATENTS 1,528,542 Hancock et a1. Mar. 3, 1925 1,832,872 Millar Nov. 24, 1931 1,837,179 Benner et a1. -2 Dec. 15, 1931 1,903,036 Francis Mar. 28, 1933 2,294,034 Jaeger Aug. 25, 1942 2,423,021 Henckler et a1. June 24, 1947 2,881,297 Friedman Apr. 7, 1959 FOREIGN PATENTS 227,223 Great Britain Ian. 15, 1925

Claims (1)

1. AN ELECTRICAL RESISTANCE FURNACE COMPRISING A HOLLOW CASING FORMED OF STRUCTURAL MATERIAL SUCH AS METAL AND HAVING GAS OUTLET MEANS AND AN INTERIOR CLOSED BY END WALLS; A HOLLOW MUFFLE FORMED OF THERMAL INSULATING REFRACTORY MATERIAL SUCH AS GRAPHITE AND ARRANGED WITHIN SAID INTERIOR AND HAVING AN INTERIOR CLOSED BY END WALLS, SAID CASING AND MUFFLE END WALLS BEING PROVIDED WITH ALINED CENTRAL APERTURES AND ALINED OUTER APERTURES SURROUNDING SAID CENTRAL APERTURES, AN ELONGATED TUBE FORMED OF DENSE REFRACTORY MATERIAL SUCH AS ALUMINA AND EXTENDING THROUGH SAID MUFFLE AND CASING, SAID TUBE FITTING TIGHTLY IN SAID CENTRAL APERTURES IN SAID CASING AND MUFFLE END WALLS AND FORMING AN ISOLATED FURNACE CHAMBER, ELONGATED SHIELDING TUBES FORMED OF REFRACTORY MATERIAL SUCH AS ALUMINA AND EXTENDING BETWEEN SAID CASING AND MUFFLE END WALLS, SAID SHIELDING TUBES FITTING TIGHTLY IN SAID OUTER APERTURES IN SAID CASING AND MUFFLE END WALLS AND PROVIDED WITH INTERIOR SURFACES OF ELECTRICAL INSULATING MATERIAL SUCH AS ALUMINA, A PARTICULAR MASS FORMED OF THERMAL INSULATING REFRACTORY MATERIAL SUCH AS CARBON AND FILLING SAID CASING INTERIOR AROUND SAID MUFFLE, FURNACE TUBE AND SHIELDING TUBES, REMOVABLE ELONGATED HEATING ELEMENTS FORMED OF ELECTRICAL CONDUCTING REFRACTORY MATERIAL SUCH AS SILICON CARBIDE AND SLIDABLY EXTENDING THROUGH SAID MUFFLE AND SHIELDING TUBES, SAID HEATING ELEMENTS INCLUDING CENTRAL HEATING SECTIONS EXPOSED WITHIN SAID MUFFLE INTERIOR IN RADIANT HEAT TRANSFER RELATIONSHIP WITH SAID FURNACE TUBE AND COLD END PORTIONS ARRANGED WITHIN SAID SHIELDING TUBES AND THEREBY ELECTRICALLY INSULATED FROM SAID MUFFLE AND MASS, AND A GAS INLET TUBE DIRECTLY CONNECTED TO SAID MUFFLE INTERIOR FOR PROVIDING AN INERT ATMOSPHERE SURROUNDING SAID CENTRAL HEATING SECTIONS AND FOR PERMITTING SAID ATMOSPHERE TO BLEED OFF AROUND SAID COLD END PORTIONS

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